1. Field of the Invention
The present invention relates to a headbox for a machine for the production of a fiber material web, and, more particularly, to a headbox having an adjustable outlet channel.
2. Description of the Related Art
A paper making machine, as is known, has a specific machine width corresponding to the desired width of a fiber material web to be produced. The fiber material web is continuously formed from the fiber suspension by applying a machine-wide fiber suspension flow onto a continuous and revolving wire belt. A headbox serves to form a fiber suspension flow which is generally uniform across the machine width so that a finished fiber material web possesses generally uniform properties across its width. The headbox includes a feed channel, a main chamber and an outlet channel. Disposed adjacent the outlet channel may be a slice beam which is movable relative to the bottom of the headbox and thereby defines a variable outlet channel. The slice beam is designed to accommodate a slow and/or fast suspension flow through the headbox.
U.S. Pat. No. 5,277,765 (Graf et al), assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference, describes a headbox having a partitioning wall disposed between two rolls within the main chamber. The partitioning wall prevents flocculation of the fiber suspension flow during low flow rate conditions within the headbox. The slice beam or movable channel wall includes an arm at each longitudinal end thereof which is connected to a stiff portion of the headbox. The arms are simultaneously used to adjust the slice beam relative to the bottom of the headbox.
In a high speed headbox, i.e., a headbox with a fiber suspension velocity at a high rate, a relatively large pressure is exerted against the bottom surface of the slice beam by the fiber suspension. If the slice beam is only supported at each longitudinal end thereof, the pressure exerted by the suspension flow across the slice beam flexes or bows the slice beam in the middle portion thereof. This results in an uneven application of fiber suspension flow from the headbox, which is undesirable.
U.S. Pat. No. 5,034,101 (Wolf et al), also assigned to the assignee of the present invention and incorporated herein by reference, discloses an apparatus for preventing flexure of the slice beam. In general, a support or reaction beam is used to provide reactionary counterforces to the slice beam and thereby prevent flexure of the slice beam. A fluid filled pressure cushion is disposed between the support beam and the slice beam and distributes the load from the slice beam to the support beam to prevent flexure of the slice beam.
U.S. Pat. No. 3,738,910 (De Noyer), assigned to Allis-Chalmers Corporation (a predecessor to the assignee of the present invention), discloses another way of adjusting the cross profile flow of fiber suspension from an outlet channel. De Noyer uses a plurality of spindles to effect a local adjustment of the slice lip across the working width thereof. Such an apparatus requires an active control system to separately adjust each of the individual spindles.
A problem with an apparatus as disclosed by De Noyer is that the adjustment of the slice beam may vary from one point to another across the machine width. Local differences in the adjustment of the slice lip may result in cross flows of the suspension, which are undesirable.
What is needed in the art is an apparatus which inhibits flexure of the slice beam, and which is relatively inexpensive and does not require an elaborate control system.